The present invention relates to a high speed light detection tube of transmission type which is used to measure scintillation in a material due to fluorescence.
A high speed light detection tube consisting of a coaxial transmission line and a photoelectric tube is proposed and practiced to measure the light beam intensity changing at high speed. Japanese Patent Applications Laid Open No. 39429/1981 and No. 39430/1981 disclosing the high speed light detection tubes have been submitted by the assignee company of the present invention.
The high speed light detection tube appearing in each of the above laid open applications uses a photocathode of reflector type. However, the high speed detection tube with a photocathode of transmission type has already been proposed.
FIG. 1 shows a cutaway view of the high speed light detection tube with a photocathode of transmission type.
Photocathode 111 of transmission type is formed within a vacuum envelope on a surface of photocathode substrate 100 made of transparent glass.
Photoelectron collection electrode 20 is formed on the top surface of cylinder 30 made of covar metal which is arranged against the photocathode 111.
The photocathode substrate 100 is fastened to cylinder 40 made of covar metal at the top thereof.
Cylinder 30 is concentrically fastened to cylinder 40 via glass seal 51 of ring type.
Cylinder 30 is fastened to glass type 61 at the other end thereof so as to keep the envelope vacuum and holes 31 are bored throughout the side wall of cylinder 30.
Electron signal currents emitted from photocathode 111 are accelerated by the electric field formed by the potential difference between photocathode 111 and photoelectron collection electrode 20, and then collected by photoelectron collection electrode 20.
The output of photoelectron collection electrode 20 can be obtained by the electrons emitted from photocathode 111 and accelerated by the electric field formed by the potential difference between photocathode 111 and photoelectron collection electrode 20.
The distance between photocathode 111 and photoelectron collection electrode 20 is set as short as possible (i.e., 1 mm) to make the rise time of the response fast in the conventional high speed light detection tube.
The light beam incident on photocathode substrate 100 is converted into photoelectrons by photocathode 111, and said photoelectrons travel in a space of 1 mm until they are collected by photoelectron collection electrode 20.
When a bias voltage of 2000 VDC is applied to photoelectron collection electrode 20 with respect to photocathode 111, the rise time measures approximately 60 ps.
If photoelectron collection electrode 20 is arranged in parallel with photocathode 111 in the high speed light detection tube of prescribed type, the current collected by photoelectron collection electrode 20 is proportional to the speed of the photoelectrons accelerated by the electric field.
Photoelectrons emitted from photocathode 111 are continuously accelerated by the DC voltage applied to photoelectron collection electrode 20 until these photoelectrons arrive at photoelectron collection electrode 20, and thus the current responding to the photoelectrons flowing through photoelectron collection electrode 20 linearly increases with time as shown in FIG. 2. This indicates that the rise time of the output current affects the increase of the induced current.
A DC high voltage is applied to the space between photocathode 111 and photoelectron collection electrode 20 so as to reduce variations in the electron transit time while photoelectrons travel from photocathode 111 to photoelectron collection electrode 20, and the span is made as short as possible (i.e., 1 mm) in such a range that no break down can occur. However, the rise time of the response sent from the conventional ultra high speed light detection tube is at most 60 ps.
The more the rise time of the response becomes shorter, the more response signal waveform of scintillation due to fluorescence becomes sharp for measuring the life of scintillation due to fluorescence.
Recently, 60 ps in the rise time of the response from the conventional ultra high speed light detector tube is too long to measure fluorescence.
The objective of the present invention is to provide the high speed light detection tube of transmission type with faster rise time.